metamorphic
Biotite Schist (with Garnet and Pyrite)
K(Mg,Fe)3AlSi3O10(OH,F)2 (Biotite focus)
Hardness: 2.5-3 (mica), 6.5-7.5 (garnet inclusion); Color: Dark brown to black with metallic gold-colored pyrite spots; Luster: Pearly to sub-metallic; Structure: Foliated/Schistose; Cleavage: Perfect basal.
- Hardness
- 2
- Color
- Dark brown to black with metallic gold-colored pyrite spots
- Luster
- Pearly to sub-metallic
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Physical properties
Hardness: 2.5-3 (mica), 6.5-7.5 (garnet inclusion); Color: Dark brown to black with metallic gold-colored pyrite spots; Luster: Pearly to sub-metallic; Structure: Foliated/Schistose; Cleavage: Perfect basal.
Formation & geological history
Formed through regional metamorphism of shale or mudstone under high temperature and pressure. Common in Precambrian and Paleozoic metamorphic belts like the Appalachians or the Alps.
Uses & applications
Used primarily in construction (road fill), insulation, as a lubricant in electrical equipment, and as a filler in various industrial plastics and rubbers.
Geological facts
Biotite is often called 'Black Mica.' The gold-colored specks visible in this specimen are likely Chalcopyrite or Pyrite, known as 'Fool's Gold,' which often precipitates in these metamorphic environments.
Field identification & locations
Identify by its platy, flaky texture that peels easily with a fingernail. It is common in mountain ranges and near tectonic boundaries. Collectors should look for large, flat mica books or garnet inclusions.
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